The Plus House—named for the perpendicular trajectories of light and air that pass through on the top and bottom floors—is one in a series of architect-designed homes commissioned by Arkitekthus, a development company founded five years ago with a pledge to improve the quality of prefab architecture. The spruce panels that coat the second-floor exterior will fade in tandem with the zinc-coated steel that rims the glazed windows and doors. "They will go gray like we do," says architect Claesson Koivisto Rune.